Salt contaminants in fossil fuels can create problems in refinery processes which can be costly to rectify. For example, these contaminants can plug downstream equipment, can form products that are corrosive to refinery equipment, and can interfere with chemical processing.
Various inorganic salts suspended in fossil fuels, such as petroleum, are usually removed by vigorous washing with water at the production site and prior to refining. There presently are three general approaches for the desalting of fossil fuels. All three require the contact of the fossil fuel with water. The selection of a particular process depends on the type of salt dispersion and the properties of the particular fossil fuel. For example, simple brine suspensions can be removed from fossil fuel by heating under pressure sufficient to prevent vapor loss [90.degree.-150.degree. C. (200.degree.-300.degree. F.)/50-250 psi], then allowing the material to settle and separate in a large vessel.
Alternatively, fresh water can be combined with the fossil fuel to form emulsions which solubilize the salts from the oil into the water. The aqueous fossil fuel emulsion can separate aided by its passage through a tower packed with sand, gravel or similar material. Emulsions can also be broken by addition of treating agents such as soaps, fatty acids, sulfonates, and long-chain alcohols. Electrostatic precipitators can employ a high potential electric field across a settling vessel to coalescence and break emulsions, in which case dissolved salts and impurities are removed with the water. As a result of the physical washing of crude oil, the salt concentration in the wash water can become quite high and unamendable to biological activity.
Sulfur contaminants of oil, as well, can be problematic. The presence of sulfur has been correlated with corrosion of pipeline, pumping, and refining equipment, and with premature breakdown of combustion engines. Sulfur also poisons many catalysts which are used in the refining and combustion of fossil fuels. Moreover, the atmospheric emission of sulfur combustion products such as sulfur dioxide leads to the form of acid deposition known as acid rain. Acid rain has lasting deleterious effects on aquatic and forest ecosystems, as well as on agricultural areas located downwind of combustion facilities. Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389. Regulations such as the Clean Air Act of 1964 require the removal of sulfur, either pre- or post-combustion, from virtually all petroleum-based fuels. Conformity with such legislation has become increasingly problematic due to both the rising need to utilize lower grade, higher-sulfur fossil fuels as clean-burning, low-sulfur petroleum reserves become depleted, and the progressive reductions in sulfur emissions required by regulatory authorities. Monticello, D. J. and J. J. Kilbane, "Practical Considerations in Biodesulfurization of Petroleum", IGT's 3d Intl. Symp. on Gas, Oil, Coal, and Env. Biotech., (Dec. 3-5, 1990) New Orleans, La.
There are no available physicochemical procedures to desulfurize crude oil, although there are several methods for reducing sulfur in refinery intermediates. One widely-used technique is hydro-desulfurization, or HDS. In HDS, the fossil fuel is contacted with hydrogen gas at elevated temperature and pressure, in the presence of a catalyst. The removal of organic sulfur is accomplished by reductive conversion of sulfur compounds to H.sub.2 S, a corrosive gaseous product which is removed by stripping. This technique cannot be applied to crude oil because of the fragile and volatile nature of some components of this material. As with other desulfurization techniques, HDS is done as a separate procedure from desalinization, requiring additional equipment.
Pretreatment desulfurization and desalting of fossil fuel prior to later refinery processing currently requires separate and distinct procedures and equipment. Various authors and inventors have proposed biological processes for the desulfurization of coal and crude oil, as a separate unit process in refineries. A need exists to develop a more efficient method for desulfurization and desalinization. This need grows progressively more urgent as petroleum companies look to cut costs in light of increased processing costs, as well as increased federal and state restrictions. Any elimination of the need for some of the equipment presently used for desulfurization and desalinization, or consolidation of the equipment used, would increase efficiency and lower costs.